The present invention relates to an image reading system such as a facsimile, and more particularly to an image reading system using a charge storing type image sensor to minimize a reading distortion.
Recently demand has been increasing for a low-cost, miniaturized image reading apparatus such as an image scanner and a facsimile, and also for a low-cost, miniaturized image sensor and motor to be used with such an image reading apparatus.
To meet this demand, a contact-type image sensor has been developed in place of a CCD. The reading period has thus been increased from 2 ms/line in a CCD scanner to, for example, 5 ms/line or 10 ms/line, and thus, the reading speed/line has been slowed down. The line means a main scanning line, called "line" hereinafter. The contact-type image sensor is a charge-storing-type image sensor which stores a predetermined amount of charge subjected to an optical electrical conversion in a predetermined period. Therefore, the amount of charge stored varies with the period. Thus, it is necessary to perform a scanning within a predetermined period.
To meet the low-cost requirement for the motor, a motor with a low response characteristic and low positional accuracy is adopted. Therefore, the reading position of the contact-type image sensor is shifted by a large amount.
In a conventional facsimile, a manuscript paper starts from a stopped state, increases its speed to a maximum and then decreases its speed until it stops. This is conducted in accordance with a motor control based on the amount of data which can be transmitted in one line.
For a more detailed explanation, the facsimile determines, based on training before a transmission of image data whether the data of the currently used line can be transmitted, for example, at 9600 bit/sec. If it cannot be transmitted at 9600 bit/sec, the facsimile further determines whether it can be transmitted at 7200 bit/sec. In accordance with such training, data is transmitted selectively at various speeds such as: 14000 bit/sec, 9600 bit/sec, 7200 bit/sec, 4800 bit/sec and 2400 bit/sec. The speed at which a manuscript paper to be transmitted is read should be variable. Thus, in a facsimile, the image data which has been read is compressed, the compression ratio varying depending on the kind of image. Furthermore, the transmission speed is not constant during reading. To prevent the buffer for storing the transmission data from being empty, and to prevent the buffer from overflowing, the speed at which the paper is transmitted should be controlled. Therefore, a stepping motor, for example, can be used to control the period of the driving pulse. An image sensor of a reading apparatus or a manuscript paper is moved a predetermined distance in the sub-scanning direction at each driving pulse.
Conventionally, the speed control of the image sensor or the manuscript paper is performed within a period given by an integer times the period for reading by an image sensor. However, when the reading period of an image sensor becomes slow, such as 5 ms/line or 10 ms/line as stated above, the reading position shift of the image sensor cannot be disregarded.
The manuscript shown in FIG. 1A is moved under the condition that a reading time period by the image sensor is at 10 ms/line scan, the pulse rate required for the stepping motor is 4 pulses/line, the sub-scanning line density is 7.7 lines/mm and the scanning speed of the reading apparatus (i.e. a reading cycle of the data on one main scanning line) is changed from 20 ms/line scan to 10 ms/line scan, for example. As recited above, the scanning speed of the reading apparatus (which is determined by the relative speed between the manuscript paper and the image sensor and is called an apparatus scan hereinafter) varies with the amount of data stored within a memory in which reading data in the reading apparatus is stored and the state of a line to which a facsimile apparatus is connected.
The reading of the pattern shown in FIG. 1A is described with regard to lines l1 to l7. The apparatus scan is at a speed of 20 ms/line from lines l1 to l3 and is at 10 ms/line between lines l3 and l7 (i.e. the speed of the apparatus scan is changed at line l3 to a high speed), the rotation speed of the motor cannot be immediately changed to 10 ms/line. As a result the positions of lines l4 and l5 are delayed as shown by lines l4' and l5' in FIG. 1B. Thus, when the manuscription speed changes, the sub-scanning line density of 7.7 lines/mm is not satisfied and the reading width is narrowed.
The image obtained by this reading operation is reproduced at a rate of 7.7 lines/mm. As shown in FIG. 1C, the portion between lines l3 and l5' of the character is extended. When the speed of the apparatus scan is reduced, the situation is reversed.
When the reading time period of the image sensor is at 10 ms/line scan, the pulse rate required for the motor drive is 4 pulses/line, the sub-scanning line density is 7.7 lines/mm, and the scanning speed of the reading apparatus is changed from 80 ms/line to 10 ms/line, then, the reading position of the image shown in FIG. 6A is as shown in FIG. 6B in a prior art apparatus, and reading lines are spaced at equal intervals in a longitudinal direction in FIG. 6B. As a result, an output is obtained as shown in FIG. 6D.
In the prior art, where an image sensor reads at 10 ms/line scan, the scanning speed of the apparatus is increased from 40 to 30 to 20 to 10 ms/line scan. Then, upon a change from 20 to 10 ms/line, a shift in reading position in which a reading is conducted, is 1/154 mm/line, then the density of a sub-scanning line is 7.7 line/mm. When the speed of an apparatus scan increases from 30 to 20 ms/line, the shift in position becomes 1/30.8 mm/line.
When a motor with a poor transient response characteristic is used, the shift in reading position is caused when the reading position is controlled by a motor at the rate of an integer times a period of reading by an image sensor, thereby decreasing the quality of the image to be output.